{"id":6020,"date":"2020-10-23T15:39:58","date_gmt":"2020-10-23T15:39:58","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/calendar-and-time-zone-in-c-20-time-zones\/"},"modified":"2023-06-26T09:37:03","modified_gmt":"2023-06-26T09:37:03","slug":"calendar-and-time-zone-in-c-20-time-zones","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/calendar-and-time-zone-in-c-20-time-zones\/","title":{"rendered":"Calendar and Time-Zones in C++20: Time-Zones"},"content":{"rendered":"

This post concludes my introduction to the chrono extension in C++20. Today I present the time-zones functionality.<\/p>\n

<\/p>\n

 \"TimelineCpp20CoreLanguage\"<\/p>\n

I should have written today I mainly present the time-zones functionality in C++20. Before I write about the time zones in C++20, I want to present the online resource Examples and Recipes <\/a>from Howard Hinnant, which has about 40 examples of the new chrono functionality. Presumably, the chrono extension in C++20 is not easy to get; therefore it’s pretty essential to have so many examples. You should use these examples as a starting point for further experiments and sharpen your understanding. You can also add your recipes.<\/p>\n

To get an idea of Examples and Recipes<\/a>, I want to present a program for Roland Bock<\/a> that calculates ordinal dates.<\/p>\n

Calculating Ordinal Dates<\/h2>\n

“An ordinal date<\/a> consists of a year and a day of year (1st of January being day 1, 31st of December being day 365 or day 366). The year can be obtained directly from year_month_day. And calculating the day is wonderfully easy. In the code below we make us of the fact that year_month_day can deal with invalid dates like the 0th of January<\/em>:” (Roland Bock)<\/p>\n

I added the necessary headers to Roland’s program.<\/p>\n

<\/p>\n

\n
\/\/ ordinalDate.cpp<\/span>\r\n\r\n#include \"date.h\"<\/span>\r\n#include <iomanip><\/span>\r\n#include <iostream><\/span>\r\n\r\nint<\/span> main<\/span>()\r\n{\r\n   using<\/span> namespace<\/span> date;\r\n   \r\n   const<\/span> auto<\/span> time =<\/span> std::<\/span>chrono::<\/span>system_clock::<\/span>now();\r\n   const<\/span> auto<\/span> daypoint =<\/span> floor<<\/span>days><\/span>(time);                    \/\/ (1)<\/span> \r\n   const<\/span> auto<\/span> ymd =<\/span> year_month_day{daypoint};         \r\n   \r\n   \/\/ calculating the year and the day of the year<\/span>\r\n   const<\/span> auto<\/span> year =<\/span> ymd.year();\r\n   const<\/span> auto<\/span> year_day =<\/span> daypoint -<\/span> sys_days{year\/<\/span>January\/<\/span>0<\/span>}; \/\/ (2)<\/span> \r\n                                                              \/\/ (3)<\/span>\r\n   std::<\/span>cout <<<\/span> year <<<\/span> '-'<\/span> <<<\/span> std::<\/span>setfill('0'<\/span>) <<<\/span> std::<\/span>setw(3<\/span>) <<<\/span> year_day.count() <<<\/span> std::<\/span>endl;\r\n   \r\n   \/\/ inverse calculation and check<\/span>\r\n   assert(ymd ==<\/span> year_month_day{sys_days{year\/<\/span>January\/<\/span>0<\/span>} +<\/span> year_day});\r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
I want to add a few remarks to the program. Line (1) truncates the current time point. The value is used in the following line to initialize a calendar date. Line (2) calculates the time duration between the two time points. Both time points have the resolution day. Finally, year_day.count()<\/code> inline (3) returns the time duration in days.\"ordinalDate2\"<\/p>\n
My following examples of time zones are also inspired by the already mentioned web resource Examples and Recipes<\/a>.<\/p>\n<\/p>\n
Times-Zones<\/h2>\n
First, a time zone is a region and its entire history of the date, such as daylight saving time or leap seconds. The time-zone library in C++20 is a complete parser of the IANA timezone database<\/a>. The following table should give you a first idea of the new functionality.<\/p>\n
\"timeZone\"<\/p>\n
I use in my examples the function std::chrono::zones_time<\/code> , which is essentially a time zone combined with a time point.<\/p>\n
Before I show you two examples, I want to make a short remark. To compile a program using the time-zone library, you have to compile the tz.cpp<\/code> file and link it against the curl<\/code> library. The curl library is necessary to get the current  IANA timezone database<\/a>. The following command line for G++ should give you the idea:<\/p>\n
<\/p>\n
\n
g++<\/span> localTime.cpp -<\/span>I <<\/span>Path to data\/<\/span>tz.h><\/span> tz.cpp -<\/span>std=<\/span>c++<\/span>17<\/span> -<\/span>lcurl -<\/span>o localTime\r\n<\/pre>\n<\/div>\n
 <\/p>\n
My first program is straightforward. It displays the UTC and the local time.<\/p>\n
UTC Time and Local Time<\/h3>\n
The UTC or Coordinated Univeral Time<\/a> is the primary time standard worldwide. A computer uses Unix time<\/a> which is a very close approximation of UTC. The UNIX time is the number of seconds since the Unix epoch. The Unix epoch is 00:00:00 UTC on 1 January 1970.<\/p>\n
<\/code>std::chrono::system_clock::now()<\/code> inline (1) returns in the following program localTime.cpp <\/code> the Unix time.
<\/code><\/p>\n
 <\/p>\n
<\/p>\n
\n
\/\/ localTime.cpp<\/span>\r\n\r\n#include \"date\/tz.h\"<\/span>\r\n#include <iostream><\/span>\r\n\r\nint<\/span> main<\/span>() {\r\n\r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n    using<\/span> namespace<\/span> date;\r\n \r\n    std::<\/span>cout <<<\/span> \"UTC  time\"<\/span> <<<\/span> std::<\/span>endl;             \/\/ (1)<\/span>\r\n    auto<\/span> utcTime =<\/span> std::<\/span>chrono::<\/span>system_clock::<\/span>now();\r\n    std::<\/span>cout <<<\/span> \"  \"<\/span> <<<\/span> utcTime <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"  \"<\/span> <<<\/span> date::<\/span>floor<<\/span>std::<\/span>chrono::<\/span>seconds><\/span>(utcTime) <<<\/span> '\\n'<\/span>:<\/span>\r\n\r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n    \r\n    std::<\/span>cout <<<\/span> \"Local time\"<\/span> <<<\/span> std::<\/span>endl;            \/\/ (2)<\/span>\r\n    auto<\/span> localTime =<\/span> date::<\/span>make_zoned(date::<\/span>current_zone(), utcTime);\r\n    std::<\/span>cout <<<\/span> \"  \"<\/span> <<<\/span> localTime <<<\/span> std::<\/span>endl;\r\n    std::<\/span>cout <<<\/span> \"  \"<\/span> <<<\/span> date::<\/span>floor<<\/span>std::<\/span>chrono::<\/span>seconds><\/span>(localTime.get_local_time()) \r\n            <<<\/span> std::<\/span>endl;\r\n\r\n    auto<\/span> offset =<\/span> localTime.get_info().offset;         \/\/ (3)<\/span>\r\n    std::<\/span>cout <<<\/span> \"  UTC offset: \"<\/span>  <<<\/span> offset <<<\/span> std::<\/span>endl;\r\n\r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
I have not added too much to the program. The code block beginning with line (1) gets the current time point, truncates it to seconds, and displays it. The call date::make_zoned<\/code> creates  std::chrono::zoned_time localTime. T<\/code>he following call localTime.get_local_time() <\/code>returns the stored time point as a local time. This time point is also truncated to seconds. localTime<\/code> (line 3) can also be used to get information about the time zone. In this case, I’m interested in the offset to the UTC.<\/p>\n
\"localTime\"<\/p>\n
My last program answers a crucial question when I teach in a different time zone: When should I start my online class?<\/p>\n
Various Time Zones for Online Classes<\/h3>\n
The program onlineClass.cpp<\/code> answers the following question: How late is it in given time zones, when I start an online class at the 7h, 13h,<\/code> or 17h<\/code> local time (Germany)?<\/p>\n
The online class should start on the 1st of February 2021, taking 4 hours. Because daylight saves time, the calendar date is essential to get the correct answer.<\/p>\n
 <\/p>\n
<\/p>\n
\n
\/\/ onlineClass.cpp<\/span>\r\n\r\n#include \"date\/tz.h\"<\/span>\r\n#include <algorithm><\/span>\r\n#include <iomanip><\/span>\r\n#include <iostream><\/span>\r\n\r\ntemplate<\/span> <<\/span>typename<\/span> ZonedTime><\/span>\r\nauto<\/span> getMinutes(const<\/span> ZonedTime&<\/span> zonedTime) {                         \/\/ (1)<\/span>\r\n    return<\/span> date::<\/span>floor<<\/span>std::<\/span>chrono::<\/span>minutes><\/span>(zonedTime.get_local_time());\r\n}\r\n\r\nvoid<\/span> printStartEndTimes(const<\/span> date::<\/span>local_days&<\/span> localDay,            \/\/ (2)<\/span>\r\n                        const<\/span> std::<\/span>chrono::<\/span>hours&<\/span> h, \r\n                        const<\/span> std::<\/span>chrono::<\/span>hours&<\/span> durationClass,\r\n                        const<\/span> std::<\/span>initializer_list<<\/span>std::<\/span>string>&<\/span> timeZones ){\r\n    \r\n    date::<\/span>zoned_time startDate{date::<\/span>current_zone(), localDay +<\/span> h};  \/\/ (4)<\/span>\r\n    date::<\/span>zoned_time endDate{date::<\/span>current_zone(), localDay +<\/span> h +<\/span> durationClass};\r\n    std::<\/span>cout <<<\/span> \"Local time: [\"<\/span>  <<<\/span> getMinutes(startDate) <<<\/span> \", \"<\/span> \r\n                                  <<<\/span> getMinutes(endDate) <<<\/span> \"]\"<\/span> <<<\/span> std::<\/span>endl;
                                                                    \/\/ (5)<\/span>
   <\/span>longestStringSize =<\/span> std::<\/span>max(timeZones, [](const<\/span> std::<\/span>string&<\/span> a, const<\/span> std::<\/span>string&<\/span> b) { return<\/span> a.size() <<\/span> b.size(); }).size();\r\n    for<\/span> (auto<\/span> timeZone:<\/span> timeZones) {                                \/\/ (6)<\/span>\r\n        std::<\/span>cout <<<\/span> \"  \"<\/span> <<<\/span> std::<\/span>setw(longestStringSize +<\/span> 1<\/span>) <<<\/span> std::<\/span>left \r\n                  <<<\/span> timeZone \r\n                  <<<\/span> \"[\"<\/span> <<<\/span> getMinutes(date::<\/span>zoned_time(timeZone, startDate))\r\n                  <<<\/span> \", \"<\/span> <<<\/span> getMinutes(date::<\/span>zoned_time(timeZone, endDate)) \r\n                  <<<\/span> \"]\"<\/span> <<<\/span> std::<\/span>endl;\r\n\r\n    }\r\n }\r\n\r\nint<\/span> main() {\r\n\r\n    using<\/span> namespace<\/span> std::<\/span>string_literals;\r\n    using<\/span> namespace<\/span> std::<\/span>chrono;\r\n    \r\n    std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n    constexpr auto<\/span> classDay{date::<\/span>year(2021<\/span>)\/<\/span>2<\/span>\/<\/span>1<\/span>};\r\n    constexpr auto<\/span> durationClass =<\/span> 4<\/span>h;\r\n    auto<\/span> timeZones =<\/span> {\"America\/Los_Angeles\"<\/span>s, \"America\/Denver\"<\/span>s, \"America\/New_York\"<\/span>s, \r\n                      \"Europe\/London\"<\/span>s, \"Europe\/Minsk\"<\/span>s, \"Europe\/Moscow\"<\/span>s, \r\n                      \"Asia\/Kolkata\"<\/span>s, \"Asia\/Novosibirsk\"<\/span>s, \"Asia\/Singapore\"<\/span>s,\r\n                      \"Australia\/Perth\"<\/span>s, \"Australia\/Sydney\"<\/span>s};\r\n\r\n    for<\/span> (auto<\/span> startTime:<\/span> {7<\/span>h, 13<\/span>h, 17<\/span>h}) {    \/\/ (3)<\/span>\r\n        printStartEndTimes(date::<\/span>local_days{classDay}, startTime, \r\n                           durationClass, timeZones);\r\n        std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n    }\r\n\r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
Before I dive into the functions getMinutes<\/code> (line 1) and printStartEndTimes<\/code> (line 2), let me say a few words about the main<\/code> function. The main<\/code> function defines the day of the class, the duration of the class, and all time zones. Finally, the range-based for-loop (line 3) iterates through all potential starting points for an online class. All necessary information is displayed thanks to the function
(line 2).<\/p>\n
The few lines beginning with line (4) calculate the startDate<\/code> and endDate<\/code> of my training by adding the start and class duration to the calendar date. Both values are displayed with the help of the function getMinutes<\/code> (line 1). date::<\/span>floor<<\/span>std::<\/span>chrono::<\/span>minutes><\/span>(zonedTime.get_local_time())<\/code> gets the stored time point out of the std::chrono::zoned_time<\/code> and truncates the value to the minute resolution. To properly align the output of the program, line (5)  <\/code><\/code>determines the size of the longest of all time-zone names. Line (6) iterates through all time zones and displays the name of the time-zone and the beginning and end of each online class. A few calendar dates even cross the day boundaries.\"onlineClass\"<\/p>\n
There is more to write about in the extended Chrono library. For example, C++20 offers new clocks such as std::chrono::utc_clock<\/code> that include leap seconds, or the std::chrono::tai_clock<\/code> that represents the International Atomic Time (TAI). Additionally, thanks to the new formatting library in C++20, time durations can be nicely formatted. This feature is not available so far. If you want to study the formatting rules for time durations, here are they: std::formatter<\/a>.<\/p>\n
What’s next?<\/h2>\n
You may have a lot of fun if you want to compare signed and unsigned integrals. This fun ends with C++20.<\/p>\n
 <\/p>\n
 <\/p><\/p>\n","protected":false},"excerpt":{"rendered":"
This post concludes my introduction to the chrono extension in C++20. Today I present the time-zones functionality.<\/p>\n","protected":false},"author":21,"featured_media":5945,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[375],"tags":[453],"class_list":["post-6020","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-c-20","tag-time"],"_links":{"self":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/6020","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/comments?post=6020"}],"version-history":[{"count":2,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/6020\/revisions"}],"predecessor-version":[{"id":9465,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/6020\/revisions\/9465"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media\/5945"}],"wp:attachment":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media?parent=6020"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/categories?post=6020"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/tags?post=6020"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}